Navigate to: C:\Users\<Your_Username>\Documents\My Games\Binding of Isaac Repentance.
Copy all .dat files (they contain your save slots—numbers correspond to save slot numbers; you may have more or fewer than three slots) into a new, separate folder.

Find your most recent save file(s)—for example, if you used slot 1, grab the file ending in "1"; if slots 1–3, grab those three. Paste those files into the root folders of both:
-Binding of Isaac Repentance
-Binding of Isaac Repentance+
This ensures the latest progress is present in both directories.

After copying your latest saves (check the date for relevance), you need to rename your file by removing the numbers from it. For example, if it was named 20250804.persistentgamedata(~).dat, you need to change it to persistentgamedata(~).dat (the name with numbers may vary slightly depending on your operating system, but in the end, you should have exactly persistentgamedata(~).dat).
P.S: (~) indicates the last digit of your save file. This is done for your convenience to accommodate different users situations.

In both the Binding of Isaac Repentance and Binding of Isaac Repentance+ folders, open the file named options.ini. Locate line 28 (near the bottom) where you'll see:
SteamCloud=1, change the 1 to 0 in both folders.

Go to:
C:\Users\<Your_Username>\Documents\My Games\Binding of Isaac Repentance+\save_backups.
Delete or move elsewhere all files in that folder.

After completing the above steps, start Repentance +. Your progress from the original REPENTANCE should now carry over successfully. Enjoy your game!

Special thanks to my friend Tundarak for highlighting the issue — without him, this guide would never have seen the light of day!
I also extend gratitude to users mrblackcat999 and Dref for their indirect participation in correcting the guide.